Interference Cancellation/Mitigation Capable Receivers
In Universal Mobile Telecommunications System/High-Speed Downlink Packet Access (UMTS/HSDPA) several interference aware receivers have been specified for the User Equipment (UE). They are termed as ‘enhanced receivers’ as opposed to the baseline receiver (rake receiver). The UMTS enhanced receivers are referred to as enhanced receiver type 1 (with two-branch receiver diversity), enhanced receiver type 2 (with single-branch equalizer), enhanced receiver type 3 (with two branch receiver diversity and equalizer) and enhanced receiver type 3i (with two branch receiver diversity and inter-cell interference cancellation capability). The new receivers can be used to improve performance, e.g., in terms of throughput and/or coverage.
In Long Term Evolution Release-10 (LTE Rel-10), enhanced interference coordination techniques have been developed to mitigate potentially high interference, e.g., in a cell range expansion zone, while providing the UE with time-domain measurement restriction information. Further, for LTE Release-11 (LTE Rel-11), advanced receivers based on Minimum Mean Square Error-Interference Rejection Combining (MMSE-IRC) with several covariance estimation techniques and interference-cancellation-capable receivers are being currently studied. In future even more complex advanced receivers such as Minimum Mean Square Error-Successive Interference Cancellation (MMSE-SIC), which is capable of performing nonlinear subtractive-type interference cancellation, can be used to further enhance system performance.
Such techniques generally may benefit all deployments where relatively high interference of one or more signals is experienced when performing measurements on radio signals or channels transmitted by radio nodes or devices, but are particularly useful in heterogeneous deployments.
However, these techniques involve also additional complexity, e.g., may require more processing power and/or more memory. Due to these factors such receiver may be used by the UE for mitigating interference on specific signals or channels. For example a UE may apply an interference mitigation or cancellation technique only on data channel. In another example a more sophisticated UE may apply interference mitigation on data channel as well as on one or two common control signals; examples of common control signals are reference signal, synchronization signals etc.
It should be noted that the terms interference mitigation receiver, interference cancellation receiver, interference suppression receiver, interference rejection receiver, interference aware receiver, interference avoidance receiver etc are interchangeably used but they all belong to a category of an advanced receiver or an enhanced receiver. All these different types of advanced receiver improve performance by fully or partly eliminating the interference arising from at least one interfering source. The interfering source is generally the strongest interferer(s), which are signals from the neighbouring cells when the action is performed in the UE. Therefore a more generic term, ‘enhanced receiver’, which covers all variants of advanced receiver, is used hereinafter. Further, the corresponding interference handling techniques (e.g., interference cancellation, interference suppression, puncturing or interference rejection combining) for enhanced receivers are termed ‘enhanced receiver technique’ herein.
Heterogeneous Deployments
In 3rd Generation Partnership Project (3GPP), heterogeneous network deployments have been defined as deployments where low-power nodes of different transmit powers are placed throughout a macro-cell layout, implying also non-uniform traffic distribution. Such deployments are, for example, effective for capacity extension in certain areas, so-called traffic hotspots, i.e. small geographical areas with a higher user density and/or higher traffic intensity where installation of pico nodes can be considered to enhance performance. Heterogeneous deployments may also be viewed as a way of densifying networks to adopt for the traffic needs and the environment. However, heterogeneous deployments bring also challenges for which the network has to be prepared to ensure efficient network operation and superior user experience. Some challenges are related to the increased interference in the attempt to increase small cells associated with low-power nodes, aka cell range expansion; the other challenges are related to potentially high interference in uplink due to a mix of large and small cells.
According to 3GPP, heterogeneous deployments consist of deployments where low power nodes are placed throughout a macro-cell layout. The interference characteristics in a heterogeneous deployment can be significantly different than in a homogeneous deployment, in downlink or uplink or both.
Examples hereof with Closed Subscriber Group (CSG) cells are given in FIG. 1, where in case (a), a macro user with no access to the CSG cell will be interfered by the Home enhanced Node B (HeNB), in case (b) a macro user causes severe interference towards the HeNB and in case (c), a CSG user is interfered by another CSG HeNB. Heterogeneous deployments, however, are not limited to those with CSG involved.
Another example is illustrated in FIG. 2, where the need for enhanced Inter-Cell Interference Coordination (ICIC) techniques for DownLink (DL) is particularly crucial when the cell assignment rule diverges from the Reference Signal Received Power (RSRP)-based approach, e.g. towards pathloss- or pathgain-based approach, sometimes also referred to as the cell range expansion when adopted for cells with a transmit power lower than neighbour cells. In FIG. 2, the cell range expansion of a pico cell is implemented by means of a parameter A. The pico cell is expanded without increasing its power, just by changing the reselection threshold, e.g., UE selects cell of pico Base Station (BS) as the serving cell when RSRPpico+Δ≧RSRPmacro, where RSRPmacro is the received signal strength measured for the cell of macro BS and RSRPpico is the signal strength measured for the cell of pico BS.
Transmit Patterns and Measurement Patterns for Enhanced ICIC (eICIC)
To facilitate measurements in the extended cell range, i.e., where high interference is expected, the standard specifies Almost Blank Subframe (ABS) patterns for eNodeBs and restricted measurement patterns for UEs. A pattern that can be configured for eICIC is a bit string indicating restricted and unrestricted subframes characterized by a length and periodicity, which are different for Frequency Division Duplex (FDD) and Time Division Duplex (TDD (40 subframes for FDD and 20, 60 or 70 subframes for TDD). Only DL patterns have been so far specified for interference coordination in 3GPP, although patterns for Uplink (UL) interference coordination are also known in prior art.
ABS pattern is a transmit pattern at a radio node transmitting radio signals; it is cell-specific and may be different from the restricted measurement patterns signaled to the UE. In a general case, ABS are low-power and/or low-transmission activity subframes. ABS patterns may be exchanged between eNodeBs via X2, but these patterns are not signalled to the UE, unlike the restricted measurement patterns.
Restricted measurement patterns (more precisely, “time domain resource restriction patterns” [TS 36.331]) are configured to indicate to the UE a subset of subframes for performing measurements, typically in lower interference conditions, where the interference may be reduced e.g. by means of configuring Multimedia Broadcast Single Frequency Network (MBSFN) subframes or ABS subframes at interfering eNodeBs.
Restricted measurement patterns may, however, be also configured for UEs with good interference conditions, i.e., receiving a measurement pattern may be not necessarily an indication of expected poor signal quality. For example, a measurement pattern may be configured for UE in the cell range expansion zone where typically high interference is expected, but a measurement pattern may also be configured for UEs located close to the serving base station where the signal quality is typically good which may be for the purpose of enabling a higher-rank transmission modes (e.g., rank-two transmissions).
Restricted measurement patterns are in general UE-specific, although it is known in prior art that such patterns may be broadcasted or multicasted. Three patterns are currently specified in the standard to enable restricted measurements:                Serving-cell pattern for Radio Link Monitoring (RLM) and Radio Resource Management (RRM) measurements,        Neighbor-cell pattern for RRM measurements,        Serving-cell pattern for Channel State Information (CSI) measurements.        
Transmit patterns and measurement patterns are means for coordinating inter-cell interference in wireless network and improve measurement performance. Alternatively or in addition to inter-cell interference coordination techniques, measurement performance may also be improved by using more advanced receiver techniques, e.g., interference suppression or interference cancellation techniques.
UE Information about Other Cells
The UE is generally aware about the serving cell(s) configuration (see also the background on multi-carrier systems). However, the UE is not only receiving/sending data and performing measurements in the serving cell(s), it may also move for which the information about neighbour cells may be helpful for mobility decisions or the network or the network and/or the UE may also perform different radio resource management (RRM) tasks and hence measurements in neighbour cells may be needed. In LTE Rel-10, the UE may receive the aggregate neighbour cell information, e.g., an indication on whether all neighbour cells use the same MBSFN configuration as the Primary Cell (PCell).
Neighbour cells lists have been mandatory for mobility and RRM purpose in earlier networks, e.g., Universal Terrestrial Radio Access (UTRA). However, such lists (comprising e.g. neighbour cell identities) are optional in LTE, and the UE has to meet the same requirements, irrespective of whether the neighbour cell information is provided to the UE or not.
Further, the UE also receives interference from neighbour cells and the UE receiver may benefit from the knowledge about the interference character (e.g., when the interfering signal occurs and where in the frequency dimension). In LTE Rel-10, to enable eICIC, the UE may receive measurement patterns via its serving cell or PCell, as described above, for measurements in the serving cell or neighbour cells. In the latter case, only one measurement pattern is provided per frequency for multiple measurement cells, together with the list of cell identities (Physical Cell Identities (PCIs)). In Rel-11, the UE should be capable to deal with even higher interference and hence even more network assistance may be needed for the UE. For example, it has been proposed that the UE should be provided the information about the number of Cell Specific Reference signals (CRS) ports and the MBSFN configuration of at least some interfering cells.
Multi-Carrier or Carrier Aggregation Concept
To enhance peak-rates within a technology, multi-carrier or carrier aggregation solutions are known. For example, it is possible to use multiple 5 MegaHerz (MHz) carriers in High-Speed Packet Access (HSPA) to enhance the peak-rate within the HSPA network. Similarly in LTE for example multiple 20 MHz carriers or even smaller carriers (e.g. 5 MHz) can be aggregated in the UL and/or on DL. Each carrier in multi-carrier or carrier aggregation system is generally termed as a Component Carrier (CC) or sometimes is also referred to a cell. In simple words the Component Carrier (CC) means an individual carrier in a multi-carrier system. The term Carrier Aggregation (CA) is also called (e.g. interchangeably called) “multi-carrier system”, “multi-cell operation”, “multi-carrier operation”, “multi-carrier” transmission and/or reception. This means the CA is used for transmission of signaling and data in the uplink and downlink directions. One of the CCs is the Primary Component Carrier (PCC) or simply primary carrier or even anchor carrier. The remaining ones are called Secondary Component Carrier (SCC) or simply secondary carriers or even supplementary carriers. Generally the primary or anchor CC carries the essential UE specific signaling. The primary CC exists in both uplink and direction CA. The network may assign different primary carriers to different UEs operating in the same sector or cell.
Therefore the UE has more than one serving cell in downlink and/or in the uplink: one primary serving cell and one or more secondary serving cells operating on the PCC and SCC respectively. The serving cell is interchangeably called as primary cell (PCell) or Primary Serving Cell (PSC). Similarly the secondary serving cell is interchangeably called as Secondary Cell (SCell) or Secondary Serving Cell (SSC). Regardless of the terminology, the PCell and SCell(s) enable the UE to receive and/or transmit data. More specifically the PCell and SCell exist in DL and UL for the reception and transmission of data by the UE. The remaining non-serving cells on the PCC and SCC are called neighbor cells.
The CCs belonging to the CA may belong to the same frequency band (aka intra-band CA) or to different frequency band (inter-band CA) or any combination thereof (e.g. 2 CCs in band A and 1 CC in band B). The inter-band CA comprising of carriers distributed over two bands is also called as Dual-Band-Dual-Carrier-HSDPA (DB-DC-HSDPA) in HSPA or inter-band CA in LTE. Furthermore the CCs in intra-band CA may be adjacent or non-adjacent in frequency domain (aka intra-band non-adjacent CA). A hybrid CA comprising of intra-band adjacent, intra-band non-adjacent and inter-band is also possible. Using carrier aggregation between carriers of different technologies is also referred to as “multi-Radio Access Technology (RAT) carrier aggregation” or “multi-RAT-multi-carrier system” or simply “inter-RAT carrier aggregation”. For example, the carriers from Wideband Code Division Multiple Access (WCDMA) and LTE may be aggregated. Another example is the aggregation of LTE and Code Division Multiple Access 2000 (CDMA2000) carriers. For the sake of clarity the carrier aggregation within the same technology as described can be regarded as ‘intra-RAT’ or simply ‘single RAT’ carrier aggregation. However, the term CA used herein may refer to any type of carrier aggregation, unless explicitly stated.
The CCs or the serving cells in CA may or may not be co-located in the same site or base station or radio network node (e.g. relay, mobile relay etc). For instance the CCs may originate (i.e. transmitted/received) at different locations (e.g. from non-located BS or from BS and Remote Radio Head (RRH) or Remote Radio Unit (RRU)). The well known examples of combined CA and multi-point communication are Distributed Antenna System (DAS), RRH, RRU, Coordinated Multi Point (CoMP), multi-point transmission/reception etc. The invention also applies to the multi-point carrier aggregation systems.
The multi-carrier operation may also be used in conjunction with multi-antenna transmission. For example signals on each CC may be transmitted by the eNB to the UE over two or more transmit and/or receive antennas.
According to Rel-11 carrier aggregation, one or more SCell can also operate on an Additional Carrier Type (ACT), which is also called as New Carrier Type (NCT). An ACT or NCT is a SCC but the cells on NCT may contain reduced number of certain type of signals in time and/or in frequency domain. For example a cell on NCT may contain Cell specific Reference Signals (CRS) only in one subframe per 5 ms. The CRS may also be reduced in the frequency domain e.g. CRS over central 25 Resource Blocks (RBs) even if cell BandWidth (BVV) is larger than 25 RBs. In a legacy carrier the CRS are transmitted in every subframe over the entire bandwidth. The SCell on NCT is therefore used for receiving data whereas important control information is mainly sent on the PCell which is transmitted on PCC. The PCC is always a normal legacy carrier i.e. contains all Rel-8 common channels and signals.
Multi-Carrier Setup or Release Procedure
A multi-carrier setup herein refers to a procedure which enables the network to at least temporarily setup or release the use of SCell, in DL and/or UL by the CA capable UE. There are two main concepts associated with the SCell setup or release and are elaborated below:                Configuration and de-configuration of SCell(s)        Activation and deactivation of SCell(s)        
Configuration and de-configuration of SCell: The configuration procedure is used by the eNode B to configure a CA UE capable with one or more SCells (DL SCell, UL SCell or both). On the other hand, the de-configuration procedure is used by the eNode B to de-configure or remove one or more already configured SCells (DL SCell, UL SCell or both). The configuration or de-configuration procedure is also used to change the current multi-carrier configuration e.g. for increasing or decreasing the number of SCells or for swapping the existing SCells with new ones. The configuration and de-configuration are done by the eNode B using Radio Resource Control (RRC) signaling.
Activation and deactivation of secondary cells: The eNode B in LTE can activate one or more secondary cells deactivated SCells or deactivate one or more SCells on the corresponding secondary carriers. The SCells which are only configured by the eNodeB can be activated or deactivated. The PCell is always activated. The configured SCells are initially deactivated upon addition and after a handover.
The network activates and deactivates the SCell(s) by sending the Activation/Deactivation Media Access Control (MAC) control element. The Activation/Deactivation command or more specifically, “Activation/Deactivation MAC Control Element (CE)” is sent via MAC to the UE. This MAC CE is identified by a MAC Protocol Data Unit (PDU) subheader as shown below:

The MAC CE has a fixed size and consists of a single octet containing seven C-fields and one R-field. The Ci and R fields in the Activation/Deactivation MAC control element are defined as follows:
Ci: if there is an SCell configured with SCellIndex i as specified in [8], this field indicates the activation/deactivation status of the SCell with SCellIndex i, else the UE shall ignore the Ci field. The Ci field is set to “1” to indicate that the SCell with SCellIndex i shall be activated. The Ci field is set to “0” to indicate that the SCell with SCellIndex i shall be deactivated;
R: Reserved bit, set to “0”.
Typically the deactivation is done when there is no data to transmit on the SCell(s) to enable UE battery saving. Currently both UL and DL SCells are activated and/or deactivated simultaneously upon receiving the MAC CE. But in principle the activation/deactivation can be done independently on uplink and downlink SCells.